This invention relates to a method and apparatus for emphasizing sharpness of an image to be used for a color scanner for mask-making, a laser color printer, a facsimile or the like.
In the prior art image scanning recording apparatus, such as a color scanner, there has been employed an electro-optical unsharpness (differential) masking method, an electrical crisping or a peaking method in order to improve the sharpness of an image. These methods are effective to improve the sharpness and reproduce a recorded image sharply, however, at the same time, since the improvement of the sharpness is effected uniformly over the whole area of an original image, the grain clusters of the original film are disadvantageously emphasized also in an area having a flat tone on the original. As a result, a poor quality of the recording image having uneven, rough graininess occurs.
In order to solve the problem described above, there has been introduced a method for emphasizing sharpness disclosed in Japanese Patent Application Laid-open No.146451/1980. In this method, a sharpness signal is formed based on a main signal obtained from a photoelectrically scanned original, or based on the main signal, and an unsharp signal which includes the information on the peripheral area of the scanned area corresponding to the main signal. This sharpness emphasis signal corrects or modifies the main signal so as to emphasize the image to be recorded. When this process is carried out, the variation level of the unsharp signal is measured or distinguished so that the quantity of the sharpness emphasis signal can be controlled on the basis of the measured variation level. In this method, no reference was made to the method of the auxiliary scanning, however, and the sharpness emphasis is carried out in accordance with only the main scanning. The lack of auxiliary scanning disadvantageously causes the deterioration of the image quality. In addition, since the unsharp signal used for the sharpness emphasis and another unsharp signal to distinguish an edge are respectively formed of the same masks having a fixed size, the above method exhibits a weakness in dealing with the image variation requirements, such as a screen line number, edge width or the like.
On the other hand, other methods for emphasizing sharpness do not have such defects as described above, such methods are disclosed by the present applicant in Japanese Patent Application Laid-open No.3269/1985 and Japanese Patent Application Laid-open No.54570/1985. However, since it is impossible for the methods to distinguish between the grain (coupler) and the edge (film; especially a reversal film), the sharpness in the flat area of the image is likely to be emphasized, which consequently leads to an unevenness of the image quality.